One apparatus for supplying respiratory gas to a patient is known from German Patent DE 34 22 066 C2. In the known ventilator, an inhalation branch and an exhalation branch communicate with one another in the form of a closed ring line. A blower, as a respiratory gas feeder unit, brings about circulation of the respiratory gas in the ring line. For controlling the phases of respiration, a triggerable inhalation valve is included in the inhalation branch and a triggerable exhalation valve is included in the exhalation branch. When the inhalation valve is open and the exhalation valve is closed, the inhalation pressure builds up in the lungs of the ventilated patient. By comparison, when the inhalation valve is closed and the exhalation valve is open, the lungs of the ventilated patient are evacuated via the intake side of the respiratory gas feeder unit. An absorber disposed in a container removes the carbon dioxide contained in the respiratory gas breathed out.
When the known ventilator is used in anesthesiology, a certain quantity of anesthetic agent is admixed with the respiratory gas. In that case, it is important that the predetermined anesthesia concentration at the patient connection be achieved as accurately as possible; that rapid changes of concentration can be attained; and that safety mechanisms are provided that give a warning if incorrect dosages are given.